Demethylation of methylbenzenes



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United States Patent DEMETHYLATION OF METHYLBENZENES Harry L. Coonradt,Woodbury, and Wilbur K. Leaman, Pitman, N. J., assignors to Socony MobilOil Company, Inc., a corporation of New York No Drawing. ApplicationSeptember 16, 1952, Serial No. 309,926

4 Claims. (Cl. 260-672) This invention relates to the conversion ofmethylsubstituted monocyclic organic compounds. It is more particularlyconcerned with the demethylation of methylbenzenes.

Frequently, during the production of aromatic compounds, e. g., inrefining of hydrocarbons there are obtained large amounts ofmethylbenzenes for which there is no immediate demand. Depending uponthe existing commercial requirements, it is desirable to convert suchmethylbenzenes to benzene or to other more desirable methylbenzenes, ingood yields and with a minimum of coking. Thus, for example, a refineryoperation may produce large amounts of toluene, at a time when there isslight market demand for toluene, but a great demand for benzene. Insuch cases, it is desirable to be able to demethylate the toluene tobenzene by means of a simple, economical process. Similarly, it is oftendesirable to convert other methylbenzenes to more desirable compounds.Thus, xylene could be converted to toluene and benzene.

As is well known to those skilled in the art, operations involving thedemethylation of methylbenzenes is much more difiicult to achieve thandealkylation operations upon benzenes having alkyl groups of two or morecarbon atoms. The catalyst employed and the reaction conditions involvedin demethylation of methylbenzenes are highly critical factors. Thus, inBritish Patent No. 637,595, it has been proposed to demethylatemethylbenzenes in the presence of a catalyst consisting of activatedalumina or of activated alumina and hydrogen chloride. It isspecifically taught therein that other catalysts, such aschromia-alumina and silica-alumina, are ineffective demethylationcatalysts.

It has now been found that the conversion of methylbenzenes into lowermethylated benzenes can be effected, in excellent yields, in thepresence of specific silicaalumina catalysts. It has now been discoveredthat methylbenzenes can be demethylated in the presence of asilica-alumina cracking catalyst containing specific amounts of silica,in the presence of hydrogen gas under pressure, and under criticaltemperature conditions.

Accordingly, it is an object of the present invention to provide acatalytic process for converting methylbenzenes to other valuablebenzenoid hydrocarbons. Another object is to provide a method fordemethylating methylbenzenes in the presence of silica-alumina catalystsin the presence of hydrogen gas. A specific object is to provide aprocess for producing benzene from methylbenzenes, such as toluene andxylenes, by contacting the methylbenzene with a bead-form silica-aluminacatalyst, in the presence of hydrogen under pressure and at specificcritical temperatures. Other objects and ad vantages of the presentinvention will become apparent to those skilled in the art from thefollowing detailed description.

The present invention provides a process for demethylatingmethylbenzenes to other valuable benzenoid hydrocarbons, which comprisescontacting a methyl- 2,786,876 Patented Mar. 26, 1957 "ice benzene witha silica-alumina cracking catalyst, containing between about 5 percentand about 20 percent alumina, the balance being silica, at a temperatureof between about 900 F. and about 1200" F., for a contact time ofbetween about 40 seconds and about 1000 seconds, and in the presence ofhydrogen gas under superatmospheric pressures.

The aromatic hydrocarbons which are demethylated by the process of thisinvention are the methylbenzenes, i. e., aromatic hydrocarbonsconsisting of a benzene nucleus having one or more methyl groupsattached thereto. In the case of the polymethylbenzenes, one or moremethyl groups can be removed, according to the desired end product.Thus, for example, xylene can be demethylated to produce either benzeneor toluene, or both. The present process is also applicable to chargestocks containing methylbenzenes in admixture with other hydrocarbons,such as highly-aromatic petroleum fractions and the like. Non-limitingexamples of the methylbenzenes contemplated herein are toluene, oxylene,m-xylene, p-xylene, hemimellitene, pseudocumene, mesitylene, prehnitene,durene, and pentamethylbenzene; and mixtures of hydrocarbons containingone or more of these compounds.

The catalyst utilizable in the process of this invention is asilica-alumina cracking catalyst. It ordinarily comprises between about5 percent and about 20 percent, by weight, of alumina, with the balance,i. e., between about 95 percent and about percent, by weight, beingsilica. Preferably, the catalyst contains between about 8 percent andabout 15 percent alumina, by weight. Also, there can be incorporatedinto the catalysts minute amounts of materials adapted to improve othercharacteristics of the catalyst. For example, minute amounts, betweenabout 0.05 percent and about 1.0 percent, by weight, of chromia can beadded to the catalyst to assist in after burning properties of thecatalyst during regeneration. This amount of chromia is not a promoter,since it does not affect the catalytic activity. Such a catalyst isstill basically a silica-alumina catalyst with respect to activity. Suchcatalysts are described in copending application Serial No. 127,626,filed November 16, 1949 now Patent No. 2,647,860. The silica-aluminacatalyst can be in any usual form in which such catalysts are used, i.e., rods pellets, spheres, etc. Preferably, however, the catalyst is inthe form of spheroidal, bead-like particles, having a particle size ofbetween about 4 and about 12 mesh. Such particles can be prepared,feasibly, in accordance with the method described in United StatesLetters Patent No. 2,3 84,946. Reference should be made thereto for theprocedures involved.

The process of this invention is effected by contacting themethylbenzene charge material with the catalyst, at elevatedtemperatures, for a relatively short period of time, in a reactionvessel suitable for carrying out catalytic reactions, and havingprovision for heat input and removal to maintain catalyst temperature.The process, of course, can be performed batchwise. Preferably, however,a continuous operation is used. In such an operation, the charge ispassed through the reactor in contact with the catalyst. Then thereaction product is subjected to a product separation operationv Theportion of the charge stock which remains undemethylated (as withpolymethylbenzenes) can be recycled to the reactor, until the maximumultimate conversion has been effected.

It has been found that the temperature of the reaction, usually measuredin terms of the catalyst temperature, is a critical factor and isdeterminative of the reaction involved. At temperatures of between about900 F. and about 1050 F., the disproportionation reaction tends to takeplace, even in the presence of hydrogen under pressure. In order toachieve demethylation, the reacgreases tion temperature must bemaintained at between about 1050 F. and about l200 F,, and preferably atbetween about 1950" F. and about l 150 F. The contact time is dependentupon the temperature involved, upon the nature of the charge material,and upon the pressure used. In general, the contact time varies,inversely with the temperature, between about 40 seconds and about 1000seconds. Preferably, the contact time varies between about 50 secondsand 500 seconds.

As mentioned hereinbefore, the present process is carried out in thepresence of hydrogen under pressure. At atmospheric pressure, theprocess proceeds to low yield, disproportionatiou, and a higher amountof coke. The pressure of the hydrogen gas must be at least about 50pounds per square inch gauge. It can be as high as about 200- pounds persquare inch gauge. The preferred range of pressures, however, is betweenabout 100 and about 500 pounds per square inch gauge.

Hydrogen can be added to the methylbenzene charge stock in molarproportions of hydrogen to methylbenzene reactant of between about 1:2,respectively, and about 10:1, respectively. In parctice, molarproportions of between about 1:1, respectively, and about :1,respectively, are preferable. The hydrogen can be in the form ofrelatively pure hydrogen gas, or it can be introduced via ahydrogen-rich gas mixture, e. g., the effiuent gas from a refineryreforming unit.

The following examples are for the purpose of demonstrating theprocesses of this invention. It must be strictly understood that thisinvention is not to be limited to the specific conditions or reactantsused in the examples, or to the operations and manipulations involvedtherein. Other conditions and charge materials, as set forthhereinbefore, can be employed, as those skilled in the art will readilyunderstand.

APPARATUS AND OPERATION The reactor used in the runs described in theexamples was a stainless steel tube suspended in a bath of molten lead.The temperature of the lead was controlled to maintain the catalysttemperature constant to within about F. Catalyst temperature wasmeasured by means of thermocouples extending into the top, middle, andbottom portions of the catalyst bed. A total volume of about 15.0 cubiccentimeters of catalyst was placed in the reactor. Accessory equipmentincluded a heated, thermostatically controlled burette for measuring thecatalyst bed at a rate sutficient to effect the desired contact time. Asample of the total gas collected was analyzed in the mass spectrometerto determine its composition and the weight of the components. Theprincipal component was methane.

The amount of coke laid down on the catalyst was determined bycombustion methods, i. e., by converting it to carbon dioxide andanalyzing therefor. The relative amounts of aromatic materials presentin a sample of the liquid products was determined by usual methods, i.e., by mass spectrometer, ultraviolet light spectrometer, distillation,etc.

REACTIONS IN THE PRESENCE OF HYDROGEN Examples 1 through 3 Runs weremade using a silica-alumina bead cracking catalyst prepared as describedin U. S. Patent No. 2,384,946. This catalyst contained about 9 percentalumina, by weight, and about 91 percent silica, by weight, and it hadan Activity Index of about 46 (a measure of catalytic activitydetermined by testing in a Cat-A unit, using the method set forth byAlexander and Shiinp, National Petroleum News, vol. 36, P. R.-537,August 2, 1944). The charge stock used in Examples 1 and 2 was toluene.Xylene was used in Example 3. Temperatures employed were 1000" F. and1100 F. with hydrogen added under a pressure of 400 pounds per squareinch, gauge. Pertinent data for these runs are set forth in Table I.

Examples 4 through 7 In order to demonstrate the effect of hydrogenpressure, a series of runs were made in which toluene was demethylatedunder various hydrogen pressures. In Examples 4, 5, and 6, there wasused a silica-alumina bead cracking catalyst containing about 9 percentalumina, by weight, and about 91 percent, by weight, silica, with about0.15 weight percent of chromia added .thereto. This catalyst, as wasdiscussed hereinbefore, is functionally the same as the catalyst used inExamples 13. The catalyst used in Exa ple 7 was the same as used inExamples 1-3. In the runs, the temperature, the L. H. S. V. and themolar proportion of hydrogen were maintained constant. The pressure wasvaried between about atmospheric pressure (Example 7) and about 400pounds per square inch gauge. Pertinent data for these runs are setforth in Table I.

TABLE I.-REAOTIONS IN THE PRESENCE OF HYDROGEN M 1 Weight Percent perPass Ultimate Weight Percent 0 es Hydro- Gata- Temp., Contact Pressure,Ha/Mole Example carbon lyst F. Time, p. s. 1. g. Hydro- Trl- Tri- FeedSee. carbon Ben- Tol- Xy-. methyl- Coke Gas Ben- Xymethyl- Coke Gus zeneuene lenes benzone lenes benzenes zones Toluene... 1,000 164 400 3.0 21.4 51. 4 14. 7 1. 4 5.0 5.0 44. 9 30. 7 3. O 10. 5 10. 5 o 1, 400 2. 443. 5 36. 4 3. 8 0.1 5. 6 9.2 70.6 6.1 1. 8 8. 9 14. 5 Xylene.... 1, 100220 400 2. 2 25. 3 33. 2 12.7 1. 0 14. 9 17. 7 27. 5 36. 2 1.0 16. 219.3 Toluene... 1, 100 153 400 3.0 40. 1 47. 9 4. 5 0. 3 4. 2 4. 7 74. 68. 5 0. 5 7. 8 8. 7 0 1, 100 a 88 200 2. 6 23. 4 64. 9 7. 1 O. 4 2. 2 5.3 61. 8 18. 6 1. 0 5. 9 13. 9 o. 1,100 44 100 2. 5 11. 3 75. 2 4. 8 0. 22. 0 2. 5 64. 3 23. 2 1. 2 9. 5 12.0 ...do. 1, 100 4. 4 5-10 2. 6 5. 684. 3 1.9 0.3 4. 9 3. 2 35. 6 12.3 0. 2 31. 6 20. 4

Silica-alumina bead eatalyst46 Activity Index-9% A1201, 91% SiOz. bSilica-alumina-ohromia bead catalyst-29 Activity Index-9% A1203, 91%SlOn+0.15% OnOs.

" Ultimate weight percent is toluene from xylene.

charge, pumps, preheater coils, a condensing and collecting system foraromatic and gaseous products, and a system for determining the amountof coke on the catalyst by a combustion method.

In operation, the catalyst, at operating temperature, was purged withnitrogen gas; followed by a flushing with hydrogen, when used in therun. Then, the charge Inateri-al, in the liquid state, together withadded gases or liquids was passed through a preheater to raise thetemperature thereof to the reaction temperature. The combined charge wasthen passed downwardly through the It Will be apra sm. from the ata setfor h n ble I, that eflicient demethylation of methylbenzenes iseffected in the presence of a silica-alumina catalyst, at temperaturesabove about 1050 F., and in the presence of hydrogen gas atsuperatmospheric pressures. Under similar conditions, but attemperatures below about 1050" F., substantial disproportionation takesplace, The foregoing examples also show that the pressure iilYQlYfldmust be greater than atmospheric pressure. In the absence of hydrogen,however, the disproportionation process becomes significant and largeamounts of coke are formed, as shown in the following examples.

REACTIONS IN THE ABSENCE OF HYDROGEN Examples 8 through 10 Runs weremade using the catalyst described in Examples 1-3, but in the absence ofhydrogen and at atmospheric pressure. Toluene was used as the chargestock for Examples 8 and 9, and xylene for Example 10. The temperaturesemployed were 1000 F. and 1100 F. Pertinent data for these runs are setforth in Table II.

Example 11 alumina cracking catalyst containing between about 5 percentand about 20 percent alumina and between about 95 percent and about 80percent silica, at a temperature of between about 1050 F. and about 1200F., for a contact time of between about seconds and about 1000 seconds,and in the presence of hydrogen gas under a pressure of between about100 pounds per square inch gauge, and about 500 pounds per square inchgauge.

2. A process for the demethylation of methylbenzenes, which comprisescontacting a methylbenzene with a silicaalurnina cracking catalystcontaining between about 8 percent and about 15 percent alumina andbetween about 92 percent and about 85 percent silica, at a temperatureof between about 1050 F. and about 1150 F., for a contact time ofbetween about 50 seconds and about 500 seconds, and in the presence ofhydrogen gas under a pressure of between about 100 pounds per squareinch gauge, and about 500 pounds per square inch gauge.

3. A process for demethylating toluene, which comprises contactingtoluene with a bead-form silica-alumina cracking catalyst containingabout 9 percent alumina and about TABLE IL-REACTIONS IN THE ABSENCE OFHYDROGEN Weight Percent per Pass Ultimate Weight Percent Con- MolesHydro- Oata- Temp., tact Pres- Hl/Mole Example carbon lyst F. Time,sure, Hydro- Tri- Tri- Feed Sec. p. s. i. g. carbon Ben- Tol- Xymethyl-Coke Gas Ben- Xymethyl- Coke Gas zene uene lenes benzene lenes benzencszenes Toluene-.. 1. 000 23 0 0 7. 8 82. 5 7. 4 0. 4 1.2 0. 7 44. 6 42. 32. 2 6.8 4.0 do. 1, 100 21 0 0 16.2 65.6 8.8 0. 5 4. 4 2. 9 49. 6 26.8 1. 6 13. 4 8. 9 Xylene..- 1, 100 26 O 0 4. 9 27. 5 36. 5 5. 2 15. 8 7.1 8. 1 45. 4 8. 6 25. 9 11. 8 Toluene"- 1, 100 175 400 2. 5 N2 37. 8 39.3 5. 1 15. 3 7. 3 57. 5 7. 7 23. 5 l1. 1

0 Ultimate weight percent is toluene from xylene.

From the foregoing examples, it will be apparent that demethylation ofmethylbenzencs is effected in excellent yields, with a minimum ofcoking, in the presence of a silica-alumina catalyst and of hydrogen gasunder pressure. When operating in the absence of hydrogen, atatmospheric pressures, disproportionation and coking are encountered.Even when pressure is used, as with an inert gas, although demethylationis improved, coking is still high.

The products produced by the process of this invention have many uses,as those skilled in the art will readily appreciate. Thus, for example,benzene can be used as a solvent and as an intermediate for theproduction of many substances, such as chlorinated benzene insecticides,phe- 1101, etc. Many halo compounds, cresols, and benzyl compounds areproduced from toluene.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationscan be resorted to without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchvariations and modifications are considered to be within the scope andpurview of the appended claims.

What is claimed is:

1. A process for demethylating methylbenzenes, which comprisescontacting a methylbenzene with a silica- 91 percent silica, at atemperature of about 1100 F., for a contact time of between about 50seconds and about 500 seconds, and in the presence of hydrogen gas undera pressure of about 400 pounds per square inch gauge.

4. A process for the demethylation of xylene, which comprises contactingXylene with a bead-form silica- 'alumina cracking catalyst containingabout 9 percent alumina and about 91 percent silica, at a temperature ofabout 1100 F., for a contact time of between about 50 seconds and about500 seconds, and in the presence of hydrogen gas under a pressure ofabout 400 pounds per square inch gauge.

References Cited in the file of this patent UNITED STATES PATENTS2,194,449 Sachanen Mar. 19, 1940 2,380,279 Welty July 10, 1945 2,384,942Marisic Sept. 18, 1945 2,620,293 Blue et a1. Dec. 2, 1952 2,647,860Plank et a1 Aug. 4, 1953 2,709,193 Clough May 24, 1955 FOREIGN PATENTS637,595 Great Britain May 24, 1950 OTHER REFERENCES Thomas et al.: I. A.C. 8., vol. 66, pages 1694-6 (1944).

1. A PROCESS FOR DEMETHYLATING METHYLBENZENES, WHICH COMPRISESCONTACTING A METHYLBENZENE WITH A SILICAALUMINA CRACKING CATALYSTCONTAINING BETWEEN ABOUT 5 PERCENT AND ABOUT 20 PERCENT ALUMINA ANDBETWEEN ABOUT 95 PERCENT AND ABOUT 80 PERCENT SILICA, AT A TEMPERATUREOF BETWEEN ABOUT 1050*F. AND ABOUT 1200*F., FOR A CONTACT TIME OFBETWEEN ABOUT 40 SECONDS AND ABOUT 1000 SECONDS, AND IN THE PRESENCE OFHYDROGEN GAS UNDER A PRRSSURE OF BETWEEN ABOUT 100 POUNDS PER SQUAREINCH GAUGE, AND ABOUT 500 POUNDS PER SQUARE INCH GAUGE.